The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with reducing fast spin echo (FSE) motion artifacts and will be described with particular reference thereto. However, it should be appreciated that the present invention may also find application in conjunction with other imaging applications which compensate for motion artifacts.
Motion artifact suppression techniques (MAST) are used to reduce ghosting artifacts produced by body motion and fluid flow. Motion artifact suppression techniques based on gradient moment nulling methods reduce motion artifacts by causing a signal produced by moving signal regions to rephase at the center of a data acquisition simultaneously with the rephasing of a signal produced by stationary signal regions. The rephasing of signals produced by moving and stationary signal regions is achieved by controlling gradient time/amplitude activity in each of the three orthogonal directions.
Motion artifact suppression techniques based on gradient moment nulling methods have proven to be an effective way to reduce motion artifacts and partially restore signal from moving tissues. MAST-based techniques have been successfully applied to a wide range of sequences. In spin echo imaging, the excitation pulse is followed by application of a refocusing pulse to induce a first echo.
However, the application of MAST techniques to fast spin echo (FSE) sequences has had drawbacks. In fast spin echo imaging, the echo is followed by another refocusing pulse to induce a second echo which is followed by a further refocusing pulse to induce a third echo, etc. The application of standard MAST techniques to FSE sequences produce less than complete motion compensation because the multiple refocusing pulses (anywhere from 2 to 256, or more refocusing pulses) produce a large number of signal pathways which experience different MAST gradient activity. The different signal pathways, with different MAST gradient activity history, constructively and destructively recombine during data readout, and produce ghosting artifacts. Modifications to the standard MAST technique have been suggested specifically for FSE sequences. The modified MAST technique attempts to synchronize the MAST gradient activity history such that there are fewer destructively interfering pathways.
Techniques which attempt to rephase additional signal pathways are better, but place even larger demands on the gradient subsystem, when compared to standard MAST methods. In general, gradient moment nulling techniques increase FSE interecho spacing, thereby reducing the image signal to noise ratio (SNR) and increasing motion artifacts compared to uncompensated FSE sequences. The SNR losses can be partially restored by increasing field of view (FOV) and slice thickness sequence minima, thereby reducing interecho spacing.
In addition, the intensive gradient activity required for gradient moment nulling may exceed amplifier limits and the gradient tube heat capacity limit for long ETL, multislice FSE sequences on some scanners, forcing additional sequence parameter compromises. The extra gradient activity also induces extra eddy currents which can degrade FSE image quality, even on self-shielded gradient systems.
It is known that for the general class of multiecho sequences, the even numbered echoes produce fewer motion related artifacts than the odd number echoes. Sequences have been designed which reduce the severity of motion-induced image artifacts by ordering the acquisition of k-space to take advantage of the even echo rephrasing phenomenon.
U.S. Pat. No. 5,431,163 discloses a number of methods for reducing motion artifacts in FSE images. In one disclosed method, motion artifacts are reduced by ordering the collection of k-space to take advantage of the even echo refocusing phenomenon. In another disclosed method, all odd numbered echos are eliminated. However, eliminating all odd numbered echos disadvantageously doubles the required scan time. In a further disclosed method, odd numbered echo 1 is retained and odd numbered echoes 3 and 5 are eliminated. However, the data acquisition windows which produce the strongest artifact, in order, are 1, 3, . . . , etc. in other words, the odd numbered data acquisition windows, with the first data acquisition window producing the strongest artifact.
Accordingly, it has been considered desirable to develop a new and improved method for reducing fast spin echo motion artifacts by echo selection which meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.